Method of reducing ignition temperature in exothermic catalytic reaction

Abstract: A method of reducing ignition temperature during exothermic catalytic reactions in gaseous reactants in a process gas being passed through a monolithic structured catalyst unit provided with a plurality of small hydraulic diameter channels (2) and a plurality of large hydraulic diameter channels (4), which method comprises passing a first portion (8) of the process gas through the small diameter channels (2) and a second portion (10) of the process gas through the large diameter channels (4), so that the first gas portion (8) flows at a low flow velocity and a high reaction rate in the unit and excess of heat evolving during the catalytic reactions proceeding at the high rate in the small diameter channels is transferred to the second portion (10) of the process gas to reduce the ignition temperature of the catalytic reactions proceeding in the large diameter channels (4).

Description: The present invention relates to exothermic catalytic reactions in gaseous reactants passing through a monolithic catalyst body. In particular, the invention is concerned with reduction of the ignition temperature during oxidation and combustion processes. Conventionally used combustion or oxidation catalysts like the catalysts in the form of pellets with a porous structure usually show inconvenient high pressure drop of a process gas. Therefore, many commercial catalytic combustors utilize monolithic supported catalysts rather than pellets. Monolithic structured catalysts having a plurality of channels provide a low pressure drop during oxidation or combustion reactions. While those reactions proceed exothermic on the catalyst, it is necessary to raise the temperature in the feed gas to a level above the ignition temperature in order to maintain the reactions proceeding in the channels of the catalyst structure. Thus, catalytic combustors are usually equipped with preheaters operating either on support fuel or electrical energy for the preheating of the feed gas. Compression of the feed gas also increases the gas temperature and partly or completely provides necessary preheating to the compressed gas. As a major drawback of preheating during catalytic processes in general, necessary supply of extraneous energy results in poorer process economy and higher investment costs for preheating equipment. Furthermore, preheating by burners causes formation of nitrogen oxides and reduces strongly the environmental advantage of catalytic combustion. This is, in particular, a problem in the catalytic combustion of gases with high ignition temperature such as methane and other hydrocarbons, which demand extensive preheating. A catalyst body with reduced back pressure resistance and improved warming up performance is disclosed in JP 63 11 3112. The body is composed of 3 monolithic sections with increasing inner peripheral surface of each monolith section in the direction of gas flow. Thus, a general object of this invention is directed to the improvement of exothermic catalytic processes being carried out in monolithic structured catalyst, by reducing the ignition temperature of the reactions proceeding on the catalyst and, thereby, substantially reducing energy consumption during those processes. Ignition temperature as used herein defines the lowest temperature at which the exothermic reactions begin and continue in a process gas passing through the monolithic catalyst body. Critical parameters, which influence the ignition temperature, comprise concentration of the reactants in the process gas and space velocity of the gas through the catalyst. Other parameters include concentration and type of the catalytic material used. As for space velocity, catalytic processes of the above art, including catalytic oxidation and catalytic combustion, are a function of this parameter since lower velocities mean longer residence time and an increased conversion rate of reactants. This in turn decreases ignition temperatures by heat evolving due to the exothermic nature of the reactions. In a monolithic structured catalyst unit, space velocity is mainly controlled by the hydraulic diameter of channels for passage of the gas at a given pressure drop across the unit. Channels with a small hydraulic diameter have a higher surface to volume ratio than large hydraulic diameter channels. Because of the higher surface to volume ratio, the catalytic surface area per unit of length is larger in the small diameter channels than in the large diameter channels. At constant pressure drop, a lower space velocity is obtained in the small diameter channels, and, therefore, the residence time of reactants in the channels and on the catalyst is much longer, which in combination with the larger catalytic surface results in much higher reaction rates than in the large diameter channels at a given gas pressure at the inlet of the catalyst unit. Based on the above considerations, this invention provides a process for the reduction of ignition temperature during exothermic catalytic reactions in gaseous reactants in a process gas being passed through a monolithic structured catalyst unit provided with a plurality of small hydraulic diameter channels and a plurality of large hydraulic diameter channels, which method comprises passing a first portion of the process gas through the small diameter channels and a second portion of the process gas through the large diameter channels, so that the first gas portion flows at a low flow velocity and a high reaction rate in the unit and excess of heat evolving during the catalytic reactions proceeding at the high rate in the small diameter channels is transferred to the second portion of the process gas to reduce the ignition temperature of the catalytic reactions proceeding in the large diameter channels. Transfer of heat from the hot process gas in the small diameter channels to reactions in the large diameter channels may be accomplished by direct or indirect heat transfer. Usage of direct or indirect heat transfer depends, thereby, mostly on the heat conducting properties of the monolithic body. A catalytic body for use in the inventive process may be prepared conventionally by corrugating e.g. metallic foils or heat resistant materials in a corrugating machine, or by extruding of ceramic material. The corrugated or extruded bodies are then loaded with catalytic active material by methods well-known in the art and include impregnation or wash coating the body with a solution containing the active material or a precursor thereof. When using ceramic carrier materials with poor heat conducting property through the channel walls, it is preferred to perform transfer of heat directly between different gas portions. Thus, in a specific embodiment of the invention, heat transfer is accomplished by mixing the hot gas portion leaving the small diameter channels and a further portion of the process gas prior to passing the mixed gas portions through the large diameter channels. The invention further provides a monolithic catalyst body (Fig. 1) being useful in carrying out the above process. The catalyst body comprises a section with small diameter 2 and a section with large diameter channels 4. In a specific embodiment using material with low heat conduction property, sections 2 and 4 are connected in series and separated by a mixing chamber 6 between the sections. In the mixing chamber, which may be equipped with conventional static mixers, a hot gas portion 8 from small diameter channels 2 is mixed with residual process gas 10 being by-passed the small diameter channels in order to reduce pressure drop in the process gas. By proper adjustment of the volume of the gas portions and the hydraulic diameter of the small diameter channels, the temperature of the mixed gas portion 12 will be at or slightly above the ignition temperature necessary for the reactions in the large diameter channels 4. A monolithic catalyst unit of the above kind may be manufactured by arranging e.g. extruded catalyst bodies with the appropriated channel size and length in a reactor tube with a mixing unit established between the bodies. In a catalyst unit being prepared from heat conducting material like metal foils, heat evolved by reaction in the small channels may be transferred by indirect exchange through the channel walls. Thus, in further a specific embodiment of the invention, a monolithic catalyst unit is provided with a plurality of small diameter and large diameter channels in parallel, so that each small diameter channel is contiguous to at least a large diameter channel. Heat is, thereby, transferred through the walls between the channel. Such catalyst unit may be prepared by corrugating sheets of suitable material with different corrugation height piling and/or rolling up the sheets to a cylindrical or sandwiched body optionally with a liner between the sheets. As in the sectional divided catalyst unit, described previously, the size of hydraulic diameters, the distribution and proportional number of small and large diameter channels is determined on the gas composition and the actual ignition temperature of reactions to be carried out in the unit. The above and other advantages of the invention will become more apparent from the following example, in which monolithic structured catalysts having channels with different hydraulic diameter are tested in the catalytic combustion of methane (eq. 1). Example The Example is a calculation model related to catalytic combustion of natural gas for gas turbine. The benefit of carrying out the combustion catalytically is a very low NO x formation, but it is required that the catalyst is able to ignite air/methane mixtures at the conditions exit the air compressor typically T = 300-400°C, and P = 10-15 atm. abs. CH 4 + 2O 2 → CO 2 + 2H 2 O The ignition temperature for this reaction depends on the resistance time, and, thereby, the linear gas velocity and the space velocity through the catalyst. For a fixed pressure drop the space velocity will decrease by decreasing the hydraulic diameter of the monolith. The ignition temperature and distance from channel inlet to the point in the channel, where the ignition temperature is obtained, are calculated for different hydraulic diameters (D h ) of 2 mm, 3.5 mm and 5 mm, respectively. Eventual heat transfer between channels has been neglected in this Example. The monolithic catalysts on which the calculations are carried out consist of a spinel carrier with outer diameter of 150 mm and a length of 1000 mm including parallel channels with the above hydraulic diameters and a wall thickness of 0.5 mm. Feed consisting of air with 3.5 mole% CH 4 was passed at the following conditions to the catalysts: Inlet pressure = 12.0 kg/cm 2 g Outlet pressure = 11.8 kg/cm 2 g Inlet temperature = 300/325/350/400°C Flow = 2300 Nm 3 /h (D h = 2.0 mm) = 5000 Nm 3 /h (D h = 3.5 mm) = 8000 Nm 3 /h (D h = 5.0 mm) For comparison, the ignition temperature is defined by 5% methane conversion. The distance for obtaining 5% conversion is listed in Table 1. D h (mm) 2.0 3.5 5.0 T inlet (°C) T ignition (°C) 300.0 344.3 16 mm 67 mm 160 mm 325.0 369.0 14 mm 58 mm 138 mm 350.0 393.7 13 mm 51 mm 120 mm 400.0 443.2 11 mm 41 mm 95 mm SV (Nm 3 /m 3 /hr) 130000. 280000. 450000. The calculations show that the necessary length, and, thereby, the size of the monolith is advantageously reduced by a factor 10 of a hydraulic diameter of 2 mm instead of 5 mm, whereas the flow through the small channels are only reduced by a factor 3.

Claims: A method of reducing ignition temperature during exothermic catalytic reactions in gaseous reactants in a process gas being passed through a monolithic structured catalyst unit provided with a plurality of small hydraulic diameter channels and a plurality of large hydraulic diameter channels, characterized by the step of passing a first portion of the process gas through the small diameter channels and a second portion of the process gas through the large diameter channels, so that the first gas portion flows at a low flow velocity and a high reaction rate in the unit and excess of heat evolving during the catalytic reactions proceeding at the high rate in the small diameter channels is transferred to the second portion of the process gas to reduce the ignition temperature of the catalytic reactions proceeding in the large diameter channels. The method of claim 1, wherein the heat is transferred directly to the second portion of the process gas by mixing the portion with the first gas portion leaving the small diameter channels. The method of claim 1, wherein the heat is transferred indirectly to the second portion of the process gas by passing the first and second portion of the gas in heat conducting relationship through the channels.

Inventors: CHRISTENSEN THOMAS SANDAHL, PRIMDAHL IVAR IVARSEN, CHRISTENSEN, THOMAS SANDAHL, PRIMDAHL, IVAR IVARSEN
Applicants: TOPSOE HALDOR AS, HALDOR TOPSOE A/S
Application year: 1994
Main classification: F01N3
IPC codes: F01N3, F01N7, F23R3, F23C99, F23C13